1. Field of the Invention
The present invention relates generally to a surgical fixation device for bone fractures, and more particularly, to a fastener and method of forming the fastener for securing a fixation plate to a bone.
2. Discussion of the Related Art
Fixation or osteosynthesis plates are used to promote the proper healing of bones especially for the realignment of separated, fractured or dislocated bone segments. Any relative movement between the bone segments may result in a failure to join the separated bone segments or an extension of time to heal the bone fracture. Therefore, it is desirable to prevent any relative movement of the bone segments at the fracture site. This involves the fixation of the separated bone segments through the use of fixation plates and fasteners to bridge the fracture and stabilize the bone segments.
Some prior art devices fasten a resorbable fixation plate to the bone segments by inserting metal bone screws through the plate and into a hole that is drilled and tapped into the bone itself. This technique provides rigid securement to the fixation plate to the bone. Examples of fixation plates which use this technique include U.S. Pat. No. 5,578,034 and U.S. Pat. No. 5,569,250.
While this technique has been used for quite some time, there are several disadvantages with resorbable fixation devices that use metal bone screws. In this regard, since a surgeon is required to both drill and tap threads into a bone, the surgeon is also required to make sure that the depth of the tapped hole and the length of the metal screw be within a precise tolerance. If the tapped hole is shorter than the screw, the screw will not seat properly and the fixation plate may not be adequately secured to the bone. This situation can be rectified by removing the screw and re-drilling and tapping the bone. However, if the screw breaks during its removal, it may require the drilling and tapping of another hole at another site or repositioning the fixation plate. Another undesirable consequence of using metal screws is the need for an additional surgical procedure to remove the screws after the bone fracture has healed.
Another approach used in the prior art to solve this problem is the use of resorbable fixation plates and resorbable screws. Examples of resorbable screws are U.S. Pat. Nos. 5,569,250; 5,290,281; and 4,550,449. However, resorbable screws are sometimes prone to cross-threading and/or breaking during insertion.
A third approach used to solve these disadvantages involves forming a fastener with a shank that is heated to a temperature that permits the material to melt and deform so as to form a tight fit within the hole drilled in the bone. An example of this approach is shown in U.S. Pat. No. 6,080,161. This fastener, however, requires a substantial amount of heat to melt and deform the shank which can cause thermal necrosis of the tissues surrounding the bone. This is undesirable and can inhibit the healing process.
The present method seeks to overcome the above mentioned disadvantages by forming a simple to use surgical securing device which after insertion into the bone, is heated to relieve uniaxial stresses in the material used to make the securing device so that the securing device distends to fill the bore and frictionally secure the fixation plate to the bone as well as reducing the incidence of thermal necrosis of the tissues surrounding the bore.
In accordance with the teachings of the present invention, a fastener and a method for forming a fastener adapted to secure a osteosynthesis plate to a plurality of bone portions are disclosed.
In the preferred embodiment, the fastener includes a head which engages an opening in the fixation plate and a shank attached to the head. The shank is disposed in a bore formed in the bone. The shank is constructed of a material having uniaxial longitudinal stresses formed therein. The stresses in the material, when heated, are relieved whereby the shank expands radially to frictionally secure the shank in the bore without deforming the material.
In the preferred method, the fastener is first formed by extruding a polymeric material. Next, the extruded polymeric material is pulformed. Then, the pulformed material is machined to form a fastener. The fastener has a head and a shank. The shank has uniaxial longitudinal stress formed therein. Next, a fixation plate is positioned so that an opening in the plate is substantially coaxial with the bore. The fastener is then disposed into the substantially coaxial bore so that the shank extends into one of the bone portions and the head overlies the fastener plate. Then, the fastener is heated to relieve the stresses in the shank whereby the shank expands radially to frictionally secure the shank in the bore.
The present invention has the advantage of providing a fastener, using pre-stressed material to secure a fixation plate to a bone portion with a bore, that is simple to use and is heated to relieve the stresses in the material in order to permit the material to expand radially to fill the bore and frictionally secure the fixation plate to the bone. As a result, the present invention reduces the incidence of thermal necrosis of the tissues surrounding the bore. Also, the disadvantages associated with the currently available surgical fasteners have been substantially reduced or eliminated.
It is therefore an object of the present invention to provide an easy to use and effective fastener for attaching a fixation plate to the bone by heating a uniaxially pre-stressed fastener to relieve uniaxial stresses in the shank and dilate the shank in order to frictionally engage the bore so that necrosis of the surrounding tissues is prevented.
It is a further object of the present invention to provide a method of forming a uniaxially pre-stressed fastener that permits securing a fixation plate to bone by annealing the fastener to relieve uniaxial stresses and to radially expand the shape of the fastener to frictionally engage the bore.